The present invention relates to a hybrid vehicle having a driving system including an internal combustion engine and another power source, a hybrid vehicle driving system, and a method of driving a hybrid vehicle.
There have been proposed hybrid vehicles having a driving system including an engine and another power source, such as an electric motor.
Hybrid electric vehicles (hereinafter abbreviated to xe2x80x9cHEVsxe2x80x9d)provided with an engine and an electric motor are classified by the type of a driving system into series HEVs (hereinafter abbreviated to xe2x80x9cSHEVsxe2x80x9d) and parallel HEVs (hereinafter abbreviated to xe2x80x9cPHEVsxe2x80x9d). In the SHEV, an engine drives a generator to generate electric energy, and a motor is driven by the electric energy to drive wheels. In the PHEV, an engine and a motor are used for driving wheels.
In a vehicle provided with only an engine, the engine is unable to operate efficiently during idling and during low-speed low-load condition, and there is a limit to the reduction of fuel consumption. A vehicle provided with only a motor must be loaded with heavy batteries for storing electricity, have a large vehicle weight and is unable to travel a long distance before the batteries are exhausted.
The HEV compensates those drawbacks in the vehicle provided with only an engine and the vehicle provided with only a motor, and is able to make the most of their advantages. During idling and low-speed low-load condition, in which the engine is unable to operate efficiently, only the motor is used, and changes the power source from the motor to the engine as vehicle speed increases.
The engine is able to operate efficiently and, when an increased torque is required temporarily for acceleration or the like, the output torque of the motor having high response characteristics is used additionally.
A HEV disclosed in Japanese Patent Laid-open No. Hei 8-294205 is provided with a lean-burn engine.
This engine is able to operate in either a stoichiometric mode in which a mixture of a stoichiometric air-fuel ratio is supplied to the engine or a lean-burn mode in which a lean mixture is supplied to the engine. This engine is controlled so as to operate in the lean-burn mode for most part of an operating time to maintain the efficiency of the engine as high as possible.
This prior art HEV, however, supplies electric energy from batteries to the motor, when the torque of the motor is necessary. Therefore, the operating mode of the engine must be changed from the lean-burn mode to the stoichiometric mode when the batteries are charged insufficiently and the engine is unable to operate continuously in the lean-burn mode.
In view of the foregoing problems, it is an object of the present invention to provide a hybrid vehicle driving system capable of making an engine mounted on a HEV operate continuously as long as possible in the lean-burn mode in which the engine is able to operate at a high efficiency.
Even if the batteries are sufficiently charged, the operating mode must be changed from the lean-burn mode to the stoichiometric mode when a high torque is required. There is a transient combustion, range in which a large amount of NOx is produced between a lean-burn range and a stoichiometric combustion range. If the operating mode of the engine is changed directly from that in the lean-burn range to that in the stoichiometric combustion range to skip an operation in the transient combustion range, the output torque of the engine changes suddenly to produce a torque shock.
To solve such a problem, it is a second object of the present invention to provide a driving method capable of making the most use of the characteristics of the HEV to add the output torque of a motor to the output torque of an engine, of changing the operating mode of the engine from that in the lean-burn range to that in the stoichiometric combustion range without producing NOx and without producing any torque shocks.
Compression ignition techniques for gasoline engines have made a rapid progress in recent years. However, any compression ignition engine capable of producing torques sufficient for driving a vehicle has not been developed.
Accordingly, it is a third object of the present invention to provide an automobile capable of producing a torque sufficient for driving a vehicle by a compression ignition engine capable of producing a low torque.
The first object of the present invention can be achieved by a hybrid vehicle driving system having a power transmitting means for selectively transmitting the output torque of an electric motor, the output torque of an internal-combustion engine or a composite torque produced by combining the output torques of the internal-combustion engine and the electric motor to driving wheels, comprising a turbogenerator capable of converting the energy of exhaust gas discharged from the internal-combustion engine into electric energy, and an electrical connecting means for electrically connecting the turbogenerator to the electric motor.
The second object of the present invention can be achieved by a hybrid vehicle driving method comprising operating an internal-combustion engine selectively in a first operating mode in which a mixture of an air-fuel ratio not lower than a predetermined first air-fuel ratio is supplied to the internal-combustion engine or a second operating mode in which a mixture of an air-fuel ratio not higher than a predetermined second air-fuel ratio and lower than the first air-fuel ratio is supplied to the internal-combustion engine, and driving wheels by a composite torque produced by combining the respective output torques of the internal-combustion engine and an electric motor, wherein the electric motor is driven and the air-fuel ratio of the mixture supplied to the internal-combustion engine is maintained at the first air-fuel ratio when the air-fuel ratio decreases below the first air-fuel ratio while the internal-combustion engine is operating in the first operating mode.
The second object can be achieved also by estimating a second air-fuel ratio engine torque that may be produced by the internal-combustion engine when a mixture of the second air-fuel ratio would be supplied to the internal-combustion engine, calculating a torque difference between an engine torque that may be produced when a mixture of the first air-fuel ratio is supplied to the engine and the estimated second air-fuel ratio engine torque, and changing the operating mode from the first operating mode to the second operating mode when the output torque of the electric motor is approximately equal to the calculated torque difference.
The third object can be achieved by a hybrid vehicle comprising an internal-combustion engine capable of operating in a compression ignition mode, a generator capable of converting the output energy of the internal-combustion engine into electric energy, and an electric motor capable of converting the electric energy generated by the generator into driving energy for driving wheels.